Valve launched its Steam Early Access program as a part of the same initiative that launched Steam Workshop and Steam Greenlight to make the Steam process much more community driven.
Steam Early Access allows users to benefit from playing a game in its development stage, whilst developers gain from player feedback in development. Developers can then make appropriate changes according to community response with iterations, and improve upon a game as much as possible before launching it as a finished product.
For now, the following titles are the only those that are available for Early Access:
· 1... 2... 3... KICK IT! (Drop That Beat Like an Ugly Baby)
· Arma 3
· Drunken Robot Pornography
· Gear Up
· Kerbal Space Program
· Kinetic Void
· Prison Architect
· Under the Ocean
The Steam Early Access tab can be found under the Games dropdown menu on the Steam front page.